Analog oscilloscopes have long been the most utilized instruments for viewing waveforms. However, with recent advancements in digital integrated circuits and processing power, digital storage oscilloscopes (DSO) have become a more viable alternative to analog oscilloscopes. In fact, DSOs offer several advantages over analog oscilloscopes, such as the ability to make automatic measurements on the digital data and the ability to store the digital data in memory for post processing viewing, generating a hard copy, uploading to a computer, or storing on a diskette. However, many engineers and technicians are resisting the transition from analog oscilloscopes to DSOs. One of the reasons that the transition to DSOs has met some resistance is the fact that analog oscilloscopes have been able to provide superior update rates. The significance of the update rate is that it indicates the number of the waveforms that are displayed per unit of time (i.e., the amount of data that is being displayed real-time). The more data displayed per unit of time, that is, the faster the update rate, the more informative is the waveform displayed by the instrument. Accordingly, manufacturers of DSOs have begun to emphasize update rate in their design criteria. However, a traditional downside to increasing the update rate in a DSO is the reduction in the number of data points captured for viewing when the user issues a stop command so as to freeze the screen for viewing the waveform statically.
For example, the display of a DSO is designed to display a fixed number of data points. The first data point represents the input waveform amplitude at the absolute time represented by the left edge of the display. The last data point represents the input waveform amplitude at the absolute time represented by the right edge of the display. If the displayed time is Td and the number of points displayed is Np, then the sample period required for the acquisition system of the DSO is given by the quotient Td/Np. Assuming the sample rate does not change, then the time required to capture X*Np points is X*Td. The number of points acquired over Np are not displayed, but represent the waveform amplitude at the absolute time to the left of the display edge and/or the absolute time to the right of the display edge. These non-displayed points are of essentially no value, until the user issues a stop command so that the waveform can be viewed in detail. When the stop command is issued, then the captured non-displayed points provide the user with more of the waveform to pan and zoom for a detailed examination.
Thus, the limiting factor in optimizing the update rate, which is preferably set at its maximum, is the time required to capture Np points. If Td is constant, then as Np increases, the update rate decreases, or becomes slower. Therefore, in order to maximize the update rate, no more than Np points should be captured for each acquisition of data. If it is desirable to capture more than Np points (e.g. X*Np), then the time required for each update becomes X*Td, thereby slowing the update rate. For instance, if Td is 1 millisecond, then the theoretical update rate limit is 1000 screen updates per second. If, however, the instrument is designed to capture 100 screens worth of data per acquisition so that a predetermined amount of data is available for detailed viewing in a stopped mode, then the update rate is only 10 screen updates per second. Thus, there is a direct trade-off between the amount of data the user is able to capture for detailed viewing and the update rate during continuous operation.
In the past, designers of DSOs have addressed this trade-off between the update rate and the size of each acquisition by either living with an undesirably slow update rate or by minimizing the number of points per acquisition. Alternatively, designers have included controls that allow the user to adjust the trade-off for a particular application. In this case, the user is provided with memory depth control so as to allow the user to select the desired memory depth. A problem with this approach is the complexity that this feature adds to the human interface with the instrument. Not only is the user required to know how to change the memory depth, the user must have a reasonable understanding of digitizing acquisition systems to fully understand the ramifications of any change. For instance, the user must be cognizant of the fact that increasing the memory depth means that the update rate will become slower. Conversely, the user must be cognizant of the fact that increasing the memory depth will make more waveform data available for viewing when close examination of the waveform is desired. In this same sense, the user must be knowledgeable enough to know that if the update rate is too slow, the user may select a smaller memory depth so that less data is captured each acquisition, thereby increasing the update rate. Lastly, it is also recognized that calculating the precise memory depth necessary to solve a particular problem which a user is attempting to solve with the DSO is no small task in and of itself, and it is oftentimes above the knowledge base of the user.
Thus, a heretofore unaddressed need exists in the industry for a DSO capable of making deep memory acquisitions for detailed examination of a waveform without having to reduce the update rate of the DSO when the instrument is continuously running.